The most widely used hydrophobic adsorbent today is activated carbon which has the advantage of large specific inner surface and a very wide heterogeneous pore spectrum. These properties are particularly useful in the purification of air and water for which such adsorbents are highly qualified.
Unfortunately, activated carbon is rather expensive and its manufacture is based on shrinking resources such as peat, low-temperature lignite and wood. The trend today, therefore, is to move away from these raw materials in favor of more abundant substances, including industrial waste products. By way of example we may mention G.D.R. (East German) patents WP No. 138,012 and No. 138,013, according to which high-temperature-lignite coke is used for the making of activated carbon, and WP No. 138,011, describing the utilization of partly carbonized ash xylite from power-plant residues as starting material for this purpose. The manufacture of an adsorbent from xylite by an alkalinic-thermal treatment is disclosed in G.D.R. patent application No. 01 J 240209/2. G.D.R. patent No. AP 85,317 describes hydrophobic mineralic adsorbents whose adsorptive effects, however, are restricted to retention of liquids immiscible with water.
Other known adsorbents are the so-called carbon molecular sieves, obtained for specific fields of use by a thermal decomposition of organic polymers, and in a broader sense the macroporous ion-exchange resins which, however, are hardly utilized for waste-water purification.
Additional drawbacks of adsorbents consisting essentially of carbon, besides their high cost, are their low temperature stability, their high affinity for oxygen and their limited regenerability.
Repeated regenerability, in fact, is an important requirement for the utilization of an adsorbent in gas and water purification. A reactivation of activated carbon laden with organic substances calls for temperatures of at least 600.degree. C. The ignition temperature of commercial coal, however, lies between 200.degree. and 400.degree. C. so that a regeneration at these high temperatures necessarily involves a loss of material which in each instance may range between 5 and 15%. This further entails a reduction in active surface of up to about 20%, which imposes a definite limit on the number of possible regenerations.